Mirror/zeek
1
0
Fork 0
mirror of https://github.com/zeek/zeek.git synced 2025-10-05 08:08:19 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 1

Change Hashing from H3 to Siphash.

Browse source 
This commit mostly changes the hash function that is used for Internal
hashing of data < 36 bytes from H3 to Siphash. This change is motivated
by the fact that it turns out that H3 apparently does not deliver a very
good source of data uniqueness; running HLL with H3 as a hashing
function results in quite poor results (up to of 75% off in my tests).
In difference, running HLL with Siphash (or HMAC-MD5) changes this
factor to ~2%.

This also fixes a long-standing bug in Hash.h which truncated our hash
values to 32 bit on most machines.

Furthermore, it once again fixes a problem with the Rank function in
HLL.
This commit is contained in:
Johanna Amann 2016-07-13 06:35:32 -07:00
parent c15f48661d
commit e1218cc7fa
10 changed files with 257 additions and 25 deletions
Download patch file Download diff file Expand all files Collapse all files

51
src/probabilistic/CardinalityCounter.cc
View file

@ -117,7 +117,9 @@ CardinalityCounter::~CardinalityCounter()
uint8_t CardinalityCounter::Rank(uint64_t hash_modified) const
{
hash_modified = hash_modified >> p;
int answer = 64 - p - fls(hash_modified) + 1;
int answer = 64 - p - CardinalityCounter::flsll(hash_modified) + 1;
assert(answer > 0 && answer < 64);
return answer;
}
@ -238,3 +240,50 @@ CardinalityCounter* CardinalityCounter::Unserialize(UnserialInfo* info)
return c;
}
/* The following function is copied from libc/string/flsll.c from the FreeBSD source
* tree. Original copyright message follows
*/
/*-
* Copyright (c) 1990, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/*
* Find Last Set bit
*/
int
CardinalityCounter::flsll(uint64_t mask)
{
int bit;
if (mask == 0)
return (0);
for (bit = 1; mask != 1; bit++)
mask = (uint64_t)mask >> 1;
return (bit);
}